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Sodium Blood Test: Low & High Sodium Levels Explained

Sodium is the primary electrolyte controlling fluid distribution. Hyponatraemia (low sodium) is the most common electrolyte disorder in hospitalised patients and requires careful systematic diagnosis.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: · How we check our content

Normal sodium
135–145 mmol/L
Hyponatraemia
<135 mmol/L
Severe hyponatraemia
<125 mmol/L — neurological risk
Most common cause
SIADH or diuretics

Approach to Hyponatraemia

StepAssessmentFinding → Diagnosis
1. Assess plasma osmolalityNormal (275–295 mOsm/kg)Pseudohyponatraemia (hyperlipidaemia, hyperproteinaemia)
2. Assess volume statusHypovolaemic (dry)Diarrhoea, vomiting, diuretics, Addison's, burns
3. Assess volume statusEuvolaemic (normal)SIADH, hypothyroidism, glucocorticoid deficiency
4. Assess volume statusHypervolaemic (oedematous)Heart failure, cirrhosis, nephrotic syndrome

Most Common Causes

  • SIADH (syndrome of inappropriate ADH): most common in hospital setting (lung cancer, pneumonia, drugs, CNS disease)
  • Thiazide diuretics, especially in elderly women
  • Heart failure and cirrhosis: dilutional hyponatraemia
  • Hypothyroidism: reduces cardiac output and free water excretion
  • Polydipsia (excessive water intake): primary or psychogenic
Rapid Sodium Correction Is DangerousCorrecting hyponatraemia too quickly causes osmotic demyelination syndrome (pontine myelinolysis): a devastating neurological complication. Maximum correction: 8–10 mmol/L in 24 hours.
Urine Sodium & OsmolalityCheck urine sodium and osmolality alongside plasma. In SIADH: urine osmolality >100 mOsm/kg and urine sodium >30 mmol/L despite low plasma sodium. This pattern confirms inappropriate ADH activity.
What are symptoms of low sodium?
Mild (130–135): nausea, headache, malaise. Moderate (125–130): vomiting, confusion, falls. Severe (<125): seizures, coma, respiratory arrest. Rate of fall matters as much as the level.
What causes high sodium (hypernatraemia)?
Almost always represents water deficit rather than sodium excess. Causes: inadequate water intake (elderly, unconscious patients), excessive sweating, diabetes insipidus, osmotic diuresis (diabetes, mannitol).
What is SIADH?
Syndrome of Inappropriate Antidiuretic Hormone: excess ADH causes the kidneys to retain water, diluting sodium. Caused by lung cancer (especially small cell), pneumonia, CNS disease, medications (SSRIs, carbamazepine, NSAIDs).
How is hyponatraemia treated?
Depends on cause and severity. Mild/chronic: fluid restrict to 1L/day (SIADH); treat cause. Severe/symptomatic: 3% hypertonic saline with maximum 8–10 mmol/L correction per 24 hours. Tolvaptan for SIADH.

Frequently asked questions

Should a borderline Sodium result be repeated?
Usually. A value just outside the interval is common in people who are entirely well, since about one in twenty healthy results falls outside any 95% reference range by construction. A borderline Sodium is normally rechecked rather than acted on.
Do Sodium results from two different laboratories compare directly?
Not reliably. Methods differ, so a change between labs can reflect the assay rather than a change in you. Where Sodium is being tracked over time, staying with one laboratory makes the comparison meaningful.
Does age or sex affect the Sodium reference range?
For several analytes it does, which is why some reports print separate intervals. Where your laboratory gives an age or sex-specific range for Sodium, that range takes precedence over any general figure.

References

The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.

  1. Hyponatremia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK470386
  2. Hypernatremia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK441960
  3. Pseudohyponatremia. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK553207

Related reading

Medical Disclaimer: This page is for general education only and does not replace professional medical advice. Always consult a qualified healthcare provider.

Sodium measures water, not salt

The most common misunderstanding about this test is that a low sodium means too little salt. In almost all cases it means too much water relative to salt. This is why eating more salt is rarely the treatment for hyponatraemia, and why fluid restriction is so often part of it. Understanding sodium as a concentration rather than a quantity makes the rest of the interpretation considerably easier.

Medications that commonly cause low sodium

  • Thiazide diuretics, the single most frequent drug cause, often within the first weeks of starting.
  • SSRIs and SNRIs, particularly in older adults, through inappropriate ADH secretion.
  • Carbamazepine and oxcarbazepine, and some antipsychotics.
  • Proton pump inhibitors and NSAIDs, less commonly but recognised.
  • Desmopressin, where the effect is a direct extension of how the drug works.

Why correction speed matters as much as the number

Sodium that has fallen slowly over weeks is tolerated far better than the same value reached in a day, because the brain adapts. That adaptation is also why rapid correction is dangerous: raising sodium too quickly in chronic hyponatraemia can cause osmotic demyelination, a severe and often irreversible neurological injury. For this reason correction is deliberately limited, typically to single figures per 24 hours, and is monitored with frequent repeat testing. It is a good example of a laboratory number where the trajectory is managed as carefully as the destination.

High sodium and who it affects

Hypernatraemia is less common and almost always reflects water loss without adequate replacement. It concentrates in people who cannot access or ask for water: infants, those with dementia, people who are sedated or immobile, and anyone with a reduced thirst response. It also occurs with diabetes insipidus and with osmotic losses in uncontrolled diabetes. Because thirst is such a powerful defence, a persistently high sodium in an alert person with free access to fluids is unusual and warrants explanation.